The terminal equipment of facsimile group 3 connectable to a communication network via analog interfaces are mainly used for the transmission of digital still images. The still pictures to be communicated via the telecommunication networks are composed of a great plurality of minute black and white mosaic squares having the same shape and size (called pixels). The original still pictures are either scanned by a scanner and resolved into black or white pixels or are generated by a computer as a result of a document/graphics production. The pixels are converted into digital code in the transmitting facsimile terminal equipment. First, they are compressed in order to reduce the quantity of information to be transmitted and, then, they are divided into information data frames and communicated block-by-block over the telecommunication network. In the receiving facsimile terminal equipment, the received information are decompressed and decoded into the original black and white pixels, an image being thus composed that is similar to the original (and is therefore displayed as a "facsimile" thereof). The receiver terminal equipment usually prints the received image out on paper or displays it in some instances on a display picture screen or stores it on local, digital storage equipment such as on a magnetic disk.
Instead of or in addition to the pixel-coded image, it is also optimally possible with terminal equipment of facsimile group 3 to send character-coded information. This service referred to as mixed mode has to operating modes, namely character-coded communication of texts and pixel-coded communication of images. It is suitable, for example, for economical transmission of letters with a letterhead and signature in fax format and with text in text format.
The described mixed mode is enabled by identifying the individual information data frames in a facsimile control field defined according to ITU-T T.4.Annex E. It is also known to send general computer data files.
The method employed by terminal equipment of facsimile group 3 has been standardized in the corresponding ITU-T recommendations for group 3 (ITU-T T.4, CCITT T.6, ITU-T T.30) (last revision 1994) according to the international telecommunication union (ITU).